There are many industrial processes which produce gaseous waste. Various chemical and mechanical techniques are known and employed to collect noxious components of the gaseous wastes.
Some circumstances arise in which some noxious or toxic components of gaseous waste exist only in very low concentrations. If a component is not particularly dangerous, a waste gas with a low concentration of the noxious substance can be safely released into the atmosphere. In some cases, however, it becomes necessary to reduce an already low concentration of a toxic component to a concentration which is still lower before a safe release can be made. For example, a waste gas may contain a very low concentration of mercury of about 1 ppmv. This concentration of mercury is still too high for safe release into the atmosphere. A concentration at more than two orders of magnitude lower is required to meet present day safety and health standards. This presents an extreme challenge for anyone who is seeking to conduct a cost-effective waste treatment operation.
The concentration of mercury is too high for safe release, but the concentration is too low to permit the use of any conventional chemical extraction techniques. One must resort to expensive and cumbersome multiple pass extraction techniques, in which a waste gas is passed time after time through a collection device as the concentration of mercury is reduced only very slightly during each pass. This requires that a collection system be extremely large or that the process generating the waste is conducted at a slow and uneconomical rate. Neither of these options are desirable.
Another vexing problem occurs when conventional thermal waste treatment systems are used. Introduction of energy needed to raise the temperature of a treated waste gas usually requires the addition of large quantities of mass to the waste gas stream. Typically, this mass addition is in the form of some combustion components such as natural gas and air or oxygen. Thermal systems thus produce an overall increase in the amount of effluent gas emerging from a system. The overall effect of this increase is higher cost and less efficient operation.
This shortcoming of thermal treatment systems has led to attempts at treating waste gases with methods that do not require or produce large scale temperature changes in the treated waste gas. These treatment techniques include such things as electrical non-equilibrium discharge systems. In these systems, there is an attempt to generate free radicals and other chemically active species through electron impact. These active species then destroy pollutant molecules through chemical reactions. Free energetic electrons are created in these prior art systems by pulsed, AC or DC corona, barrier type discharges and electron beam injection. A few examples of such systems are described in U.S. Pat. Nos. 3,983,021 (J. M. Henis) and 4,954,320 (J. G. Birmingham et al.).
These systems perform satisfactorily within certain limits. In many cases the amount of energy which can be applied to purging a gas of pollutants is limited. Electrical discharge systems are efficient only when they operate in a non-arcing mode. In order to operate in such a non-arcing mode, these systems must be operated with low current levels or in a pulsed mode. Because of high spatial non-uniformity of these discharges, one finds that transformation of electric energy into useful chemical reaction is not very efficient. Consequently, it is difficult to transfer large amounts energy into a fast moving stream of waste gas.
In addition to treatment of waste gases, there are other instances in which there is a need to remove or extract contaminants from a gas. For example, in the semiconductor industry, there is a need to provide very high purity silane to particular manufacturing operations. When contaminants within a gas are reduced to levels below a range of a few parts per billion, it becomes very difficult to produce further reductions. However, in some applications it is necessary to reduce contaminants to concentration levels that are two to five orders of magnitude lower than one part per billion.
Prior art purification techniques relied on chemical processes that were dependent on diffusion as a transport mechanism. When extremely low concentrations of an impurity exist, diffusion based chemistry is a very slow process. Consequently, prior art purification systems required complex, multiple stage equipment with large reactor surface areas.
In the prior art, some efforts have been made to achieve purification of waste gases or other industrial gases through use of charged particles. Various types of prior art corona or other discharge devices are used to produce charged particles in applications such as light sources and treatment of surfaces. Some attempts were made in the prior art to apply these devices to removal of impurities from various gases.
In such applications, a desirable feature of any discharge device or system is an ability to convert a large portion of the energy supplied to the system into useful currents of charged particles. Another desirable feature of such systems is that they provide a desired current of charged particles with an application of voltage that is relatively low, so that commonplace dielectric materials can be used for their construction.
Throughout the prior art, an achievement of these design goals has not been fully attained.
In most corona discharge systems, there is a limiting set of operating conditions which, when exceeded, results in formation of arcs and streamers. Typically, arcs and streamers are not a useful form of output energy for these devices.
In order to avoid production of arcs and streamers, prior art discharge systems are operated with relatively low saturation currents. In a typical prior art corona discharge device, a driving voltage of 2500 volts will produce a current of about 0.001 mA. An increase of voltage to 10,000 volts will produce a current that is only about one order of magnitude higher.
Prior art discharge devices produce charged particles of various types. They produce particles such as ions, free electrons and various free radicals. Typically, these particles are produced without any selectivity. In some instances, it is desirable to use particles of a particular type to conduct a process. For example, we have found that some processes such as refinement of gases are most efficiently conducted by using ions.
It is a goal of the present invention to provide a method of producing charged particles with a high ratio of particle current to applied voltage.
It is a goal of the present invention to produce charged particles with a selectivity that provides a predominantly ions.
It is a goal of the present invention to provide a method of treating contaminated gases with a discharge system which permits efficient introduction of energy into a the gas.
It is a further goal of the present invention to provide such a method which has a capability to extract pollutants that are present in the gas in very low concentrations.